As mobile communication has become increasingly critical to the conduct of business, a need has arisen for smaller and more compact telephones. Until now the size and shape of the telephone has been driven in large part by the size of the circuitry needed and by the need to maintain a certain separation between the telephone mouthpiece and the earpiece. Recent developments in integrated circuit technology have driven down the size of the telephone circuitry. According to Olsen, U.S. Pat. No. 4,847,818 issued Jul. 11, 1989 to Olsen and Blonder, U.S. Pat. No. 5,239,521 issued Aug. 24, 1993, however, even if the telephone unit can be sufficiently integrated to fit within a compact body, the telephone microphone and the telephone speaker must still be spaced a certain minimal distance apart in order to prevent feedback from the speaker through the microphone.
Feedback is inherent to telephone design. In the typical telephone, some feedback of the microphone output back through the speaker (in the form of a sidetone) is necessary to serve as an indication to the user of how loudly he or she should speak. However, this feedback can cause problems if it, in turn, is coupled back from the speaker through the microphone. The resulting feedback loop can lead to oscillations heard as an audible hum or howl. Spatial separation reduces the amount of signal coupled from the speaker to the microphone and therefore prevents oscillation.
In compact telephones, such as a wrist-mounted telephone, the spatial separation constraint forces the manufacturer to go through elaborate contortions to place the microphone and/or speaker as satellites attached to the telephone unit body. In Olsen, the speaker is placed at the clasp of the watch while the microphone is either located at the opposite clasp or built into the body of the telephone unit. In Blonder, a special pivoting member is used to separate the microphone from the speaker. Finally, in Yoshitake, U.S. Pat. No. 5,008,864 issued Apr. 16, 1991, a speaking tube is used to provide directionality to the telephone microphone.
However, none of the above solutions is very practical. Olsen and Yoshitake both require that the user remove the watch before placing or receiving a telephone call. Blonder requires the addition of a separate layer attached to the wrist strap that can pivot around the watch band. Such an approach drives up manufacturing costs and reduces reliability.
The feedback problem has also been addressed in the context of speaker phones. Speaker phones will howl if feedback of the sidetone from the speaker through the microphone reaches too high a level. This problem can be controlled by turning the speaker off during times when the user is speaking and by reducing the gain of the microphone amplifier during periods when the speaker is on. Another approach controls oscillation by reducing gain in the speaker amplifier during periods when the user is speaking and then reducing gain in the microphone amplifier during periods when the user is quiet. Since both approaches rely on the accurate detection of whether the user is speaking, they can cause audible discontinuities in both the received and transmitted voice transmissions at the start and end of user speech.
In addition to the feedback problem, portable radio telephones to date, especially those designed to be worn on the wrist, suffer from a number of deficiencies. The power sources for portable radio telephones tend to be short-lived and difficult to replace. Typically, the entire telephone unit must be placed on a recharging stand, rendering it unavailable to the user. Alternatively, the radio telephone must be shut-off to replace the battery.
Also, when a user cannot use the speaker phone because of the need for privacy, a separate earpiece must be located and carefully attached to the phone unit. Separate earpieces are difficult to engage with the phone and easy to misplace.
Additionally, key pads on portable radio telephones, similar to other miniaturized electronic devices such as calculator watches, typically are too small to be operated by hand. The user typically must utilize a pen or other pointed object to activate a particular key.
It is clear that there is a need in the art for a compact wireless telephone which will minimize feedback while providing an acceptable voice signal. In addition, there is a need in the art for a compact wireless telephone which includes an ergonomic keypad, a retractable earpiece attached to the wireless telephone and a power supply which is easy to replace yet long-lasting.